Equol, the major metabolite of the phytoestrogen daidzein - one of the main isoflavones found abundantly in soybeans - completely stops in its tracks the male hormone dihydrotestosterone (DHT), which normally stimulates prostate growth and causes male pattern baldness.
Several human studies have suggested that eating soy reduces the risk of prostate cancer. This new study could explain why, and also lead to treatments for other hormone-related diseases in men.
"These findings are of immense clinical importance because blocking the action of the potent androgen (male hormone) DHT has been one of the holy grails of the pharmaceutical industry as a strategy for treating prostate cancer and other related diseases," said Kenneth Setchell, from the Cincinnati Children's Hospital Medical Center, who first identified equol in humans 20 years ago.
In recent years, the pharmaceutical industry has developed drugs that inhibit a certain enzyme that converts testosterone to DHT. Unfortunately, these drugs have side effects. Equol, on the other hand, does not prevent DHT from being made but prevents it from functioning by stopping it from binding to the androgen receptor and thereby preventing the prostate from growing.
This may be particularly important for men who have been diagnosed with either an enlarged prostate (benign prostatic hyperplasia, or BPH) or cancer of the prostate.
"Directly binding and inactivating DHT without influencing testosterone gives equol the ability to reduce many of the harmful effects of androgens without affecting the beneficial ones," said Robert J. Handa, senior author of the study, published in the April edition of Biology of Reproduction(70: 1188-1195).
"The novelty of equol is that it both inhibits androgen hormone and influences oestrogen hormone action. We do not know of any other molecule that possesses these important biochemical properties," added Edwin Lephart, director of the Neuroscience Center at Brigham Young University, which also participated in the research.
Two experiments demonstrated that injections of equol into male rats reduced the size of the prostate. In one study, the testes of male rats were removed, thereby eliminating all DHT production. When investigators injected DHT into rats, their prostates grew. When they gave rats equol, nothing happened at all. When they injected rats with both equol and DHT, the equol prevented the DHT from functioning as it normally would - as a stimulator of prostate growth.
In other words, equol did not change hormone levels but completely blocked the effects of DHT in rats. This could explain why men in Japan, who eat more soy than men in the Western world but suffer equally from BPH as they age, rarely go on to have prostate cancer, said Dr Setchell.
The researchers added that, given the importance of DHT in the skin, it is possible that equol may offer a means of controlling hair loss and promoting healthy skin. They have started further studies to assess equol's potential as a treatment for a variety of other androgen-mediated conditions.
The team has filed patent applications on equol and hopes to commercialise the technology.